Application of statistical inference to optical time domain reflectometer data

ABSTRACT

The present invention relates to method for interpreting data obtained by measuring a length of optical fiber using an optical time domain reflectometer (OTDR), and comparing that measurement to a reference measurement. The technique uses statistical inference to determine a likely cause for a length measurement to be shorter than a reference length. One technique uses a chi-squared best fit of an array reflectance spike occurrences along the fiber to a historical reference array. In that way, it can be determined whether the missing portion of the tested fiber is at an end or between the ends, providing evidence that the short length measurement results from a fiber break or from the intentional removal of a reserve loop.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/897,929,entitled “Application of Statistical Inference to Optical Time DomainReflectometer Data,” filed on Jul. 23, 2004 now U.S. Pat. No. 6,989,893.

FIELD OF THE INVENTION

The present invention relates generally to optical networktroubleshooting. Specifically, the invention provides a method forinterpreting an optical fiber measurement trace taken by an optical timedomain reflectometer (OTDR) using statistical inferences.

BACKGROUND OF THE INVENTION

Telecommunications network operators rely heavily on the integrity oftheir fiber cable networks in competing for customers on the basis ofquality of service. Carriers can no longer tolerate service outages oncables, or even single fibers, designed to transport numerousgigabit-per-second optical channels.

Once an optical cable has been installed, network providers must becertain that each separate fiber span matches or exceeds the carrier'sspecifications. Testing and troubleshooting of in situ optical fibercable is frequently carried out using optical time domain reflectometer(OTDR) instruments, such as the Series FTB-7000B ODTR sold by EXFOElectro-Optical Engineering Inc. of Vanier (Quebec) Canada.

The OTDR characterizes fibers at a high level of detail, generatingdistance versus attenuation data, as well as insertion loss measurementsfor all splices, defects, kinks, and breaks. An OTDR functions byinjecting a short, intense laser pulse into the optical fiber andmeasuring the backscatter and reflection of light as a function of time.A simplified, schematic representation of an OTDR 110 is shown inFIG. 1. The OTDR includes a laser source 116 and a detector 117, eachconnected to a subject fiber 100 via a coupler 118. Laser energy isinjected into the fiber 100 in the form of hundreds of pulses persecond. A portion of the laser energy travels to the fiber termination,but splices, breaks, bends and any other anomalies in the fiber reflectsome portion of the laser energy. Those characteristics can be locatedalong the fiber by observing the round trip transmission time of thereflected laser energy. The characteristics are measured thousands oftimes and averaged to increase accuracy.

The reflected light characteristics are analyzed to determine thelocation of any fiber optic breaks or splice losses. FIG. 2 shows asample output trace of a typical OTDR. Distance along the fiber cable isrepresented in kilometers on the x-axis 210, and attenuation 220 of theLaser signal is represented in decibels on the y-axis 212.

Many of the characteristics seen on an OTDR trace are the result ofFresnel reflectance caused by abrupt changes in the index of refraction(ex: glass/air). For example, the mechanical splice connector 221 andspan end 223 shown in the trace of FIG. 2 are the result of glass/airinterfaces. Using the location of the span end 223 on the x-axis, alength of the fiber span may be accurately calculated. A fusion splice222 eliminates a glass/air interface, but it nevertheless generates asubstantial level of reflected signal power as compared to thebackscatter level, and is detectable on the trace. Floor noise 224 maybe seen at distances beyond the termination 120 (FIG. 1) of the fiber.

The proper interpretation of OTDR data—whether generated manually orautomatically—frequently depends upon its comparison to one or morereference traces. A reference trace, however, is a static snapshot of afiber that quickly becomes out of date, both from aging characteristicsof the optical material, and as a result of network maintenance.

For example, a test OTDR trace may be compared to a reference trace, andthe test trace may reveal a shorter overall fiber length than that shownby the reference trace. That situation raises the question whether theshorter fiber length is due to an accidental cable cut, or is due to acable loop being removed in the course of normal network operations.Similarly, if a test trace displays a higher loss reading, the questionarises whether this an equipment fault, or is simply aging of the fiber.

A wide body of tools exists in the industry to analyze a test trace, andto compare that trace to an assumed good reference. However, working inthe reverse direction—comparing the reference to the newer testtrace—gives a significant advantage to the process.

There is presently a need for a set of statistical tools to increaseconfidence in the comparison and analysis of OTDR data. Those toolsshould permit the automatic detection of certain conditions such as:

Reference Trace is invalid and must be updated; and

Reference Trace is valid, but test trace is suspect.

Even if both traces are valid, the toolset should allow additionalinformation to be extracted from the test trace, such as a possiblefuture failure type or failure point.

SUMMARY OF THE INVENTION

The present invention addresses the needs described above with a methodfor analyzing a measurement of reflective characteristics in an opticalfiber. Parameters are received from a test OTDR trace T_(test) of theoptical fiber, the parameters including a gross distance measurementD_(test) and at least one array parameter P

_(test). Parameters from a reference OTDR trace T_(ref) are retrievedfrom a memory, the parameters including a gross distance measurementD_(ref) and at least one array parameter P

_(ref). D_(ref) and D_(test) are compared, and if D_(ref) is greaterthan D_(test), then correlating fiber characteristics represented by P

_(ref) with fiber characteristics represented by P

_(test) to determine a location of a portion of fiber represented by P

_(ref) that is not represented by P

_(test).

If D_(ref) is less than D_(test), then D_(ref) may be replaced in thememory with D_(test). The step of correlating fiber characteristics mayuse an inferential statistical method. The inferential statisticalmethod may include performing a chi-squared best fit of the parameter P

_(test) against the parameter P

_(ref).

The method may also comprise the step of selecting a significance levelα for the correlation of the fiber characteristics.

The parameter P

_(test) may be an array R

of reflectance spikes detected along the fiber. If D_(ref) is notstatistically significantly greater than or less than D_(test), and areflectance array R

_(test) from the test trace T_(test) varies statistically significantlyfrom a reference reflectance array R

_(ref), then D_(ref) may be replaced in the memory with D_(test).

The parameter P

_(test) may be an array IL

of instantaneous loss values measured along the fiber, or mayalternatively be an array GL

of gross loss values for points along the fiber.

The step of comparing D_(ref) and D_(test) may comprise the steps ofassigning an experimental error σ associated with the measurement ofD_(ref) and D_(test); if D_(test)+2σ>D_(ref) then conclude that D_(test)is less than D_(ref); if D_(test)>D_(ref)+2σ then conclude that D_(test)is greater than D_(ref); and if D_(test)<D_(ref)<D_(test)+2σ thenconclude that D_(test) may be equal to D_(ref).

If D_(ref) is less than D_(test), then D_(ref) may be replaced in thememory with D_(test).

In another embodiment of the invention, a machine readable medium isprovided containing configuration instructions for performing the abovemethod.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an OTDR system connected to a fiber cable.

FIG. 2 is a sample graphical output from an OTDR system.

FIG. 3 is a schematic diagram showing a system utilizing one embodimentof the invention.

FIG. 4 is a flow chart depicting a method according to an embodiment ofthe invention.

DESCRIPTION OF THE INVENTION

The present invention provides a process for using a test OTDR trace toeither validate or refute an assumed good reference trace. The processalso allows additional determinations to be made about the validity ofthe test trace, and to refine error analysis in an alarm situation.

A block diagram of an exemplary system 300 in accordance with oneembodiment of the invention is shown in FIG. 3. Raw measurement data istransmitted from the laser detector 310 to an OTDR processor 320. In onetypical OTDR instrument, the processor is integral with the laserdetector, the laser and the coupler (FIG. 1) and is programmed todetermine fiber characteristics based on the test trace. The OTDRprocessor 320 may have integral memory (not shown) for storing pastmeasurement results.

Test trace characteristics are transmitted from the OTDR processor 320to a statistical post-processor 330. The term “statisticalpost-processor” as used herein does not preclude statistical processingfrom being performed in the OTDR processor as well. Indeed, themeasurement averaging and other statistical functions used incalculating the trace characteristics are typically carried out in theOTDR.

The statistical post-processor 330 may be part of the OTDR processor ormay be a separate processor residing, for example, in a desk-topcomputer. The statistical post-processor 330 has access to a machinereadable medium 340 on which are stored instructions that, when executedby the processor, perform the method of the invention. The machinereadable medium may be removable media such as an optical disk ormagnetic disk, or may be fixed magnetic disk. The medium mayalternatively be internal volatile or non-volatile memory.

In executing the process of the invention, the statisticalpost-processor accesses memory 350 to recall reference trace data orreference test trace characteristics stored on that memory. The memory350 may reside within the statistical post-processor 330 or within theATDR processor 320. The reference trace characteristics are used forcomparison to the test trace characteristics to detect changes in thefiber.

Certain statistical characteristics of each of the trace data sets areinitially computed for the Test trace (T_(test)). If thosecharacteristics are not already available for the Reference Trace(T_(ref)), then they are computed for that trace as well. For example,the following values may be computed:

D Gross Distance (overall length of the fiber tested)

R

array of Reflectance spikes (inflection points) found along the fiber

GL

array of Gross Loss values for all points along the fiber

IL

array of Instantaneous Loss values (rate of loss) for points along thefiber

DV

array Divergence Values between the two traces (IL

_(ref)−IL

_(test))

The gross distance or length values are computed by determining thelocation along the fiber of the span end spike. For example, in thetrace shown in FIG. 1, the fiber has a length value of approximately 10km to the span end 223. The spike at the span end is often fairly welldefined in the trace because it appears as a Fresnel reflection of theglass/air interface of a polished fiber end.

The array parameters R

, GL

, IL

and DV

are arrays containing values for a given characteristic at a pluralityof points along the fiber optic cable.

In a method of the invention 400, shown in FIG. 4, the gross distance,or length, calculated from the test trace is compared (step 405) to thegross distance calculated from the reference trace.

In comparing values such as fiber length for equality or inequality, anexperimental error (σ) may be taken into account. A reading of D impliesan actual value of |D+/−σ|. So, to compare D₁ and D₂, the followingmulti-valued logic must be used:

D₁+2σ<D₂: D₁ is definitely less than D₂

D₁>D₂+2σ: D₁ is definitely greater than D₂

D₁<D₂<D₁+2σ: D₁ and D₂ may or may not be equal

Determinations of equality and relative magnitude made in the presentlydescribed method are preferably made using a similar multi-valued logic.A value of the experimental error σ may be calculated from themeasurement data, or may be estimated based on past performance.

Based on the relative calculated values for the gross distance D_(test)of the test trace T_(test) and the gross distance D_(ref) of thereference trace T_(ref), three possible scenarios exist; each of thosescenarios is examined separately in a preferred method according to theinvention.

The case (decision block 420) in which D_(ref)<D_(test) is the moststraightforward case. If the test trace identifies a longer fiberlength, then the test trace is adjudged to be superior to the existingreference trace, the assumption being that the only possible explanationis the (intentional) physical installation of additional cable. In thatcase, the reference trace T_(ref) may be replaced (step 421) with thetest trace T_(test), which is determined to be more current.

Still, a χ² (chi-square) standard deviation test (not shown) on T_(ref)versus the initial length-matched portion of T_(trace) can be performed.That additional test gives a confidence factor in the test trace; afailure indicates the trace is likely invalid.

Another possible case is the case (decision block 410) whereD_(ref)>D_(test). That is a more complex case. While the underlyingnetwork has undisputedly changed, it is necessary to determine whetherthe new (shorter) cable more or less functional. The cable may beoptically shorter due to an accidental cut or partial failure (negativeimpact). Conversely, a loop or splice section may have been removedintentionally during maintenance (neutral/positive impact). Essentially,the technique attempts to discover which is a more accurate picture ofthe desired network—T_(ref) or T_(test).

To establish which of the above alternatives is most likely, thefollowing assumption is made: if the section missing from the newer(test) trace lies between both endpoints, then the change is assumedintentional. Stated conversely: an accidental cut will cause adiscontinuity between the endpoints of a cable, whereas maintenanceoperations preserve end-to-end continuity.

Given that assumption, it is possible to determine the cause of thelength discrepancy by establishing a correlation (step 411) between thefiber characteristics of various sections of T_(ref) and T_(test). Anattempt is made to make both a positive correlation (the missing sectionis definitely at the far end, indicating a likely fiber cut) and anegative correlation (the missing fiber is definitely not an endpoint,indicating a maintenance-related intentional shortening).

A positive correlation can be tested (step 412) by one of severalinferential statistical methods. Essentially, a two-tailed hypothesistest is desired, with null hypothesis T_(test)=(segment of) T_(ref). Thesignificance level (α) of the match can be tuned by application andsituation.

For instance, a chi-squared best fit can be performed against R

_(test) and the initial length-matched portion of R

_(ref). A successful test indicates T_(ref) as valid; in practice, sucha result would indicate a likely fiber break (step 414). A failure leadsus to reject the null hypothesis and assume T_(test) to be superior(step 413).

In the absence of data for R

(i.e. a fiber section either without splices, or with a very smallsample set of splices), the same correlations can be performed on GL

and IL

, though a correspondingly higher confidence factor must be chosen.

If the relative magnitude of D_(ref) and D_(test) cannot be resolvedwithin statistical bounds, and the optical path length remainsstatistically unchanged, there are still opportunities for extractingdata on the relevance of T_(ref). Any statistically significant variancein R

yields the conclusion that the reference trace is dated. Variances in GL

or IL

are less meaningful, but may still trigger an exception if a presetvalue is exceeded.

If, through any of the above statistical determinations, the test traceis adjudged superior, various possibilities exist for remedial action.An alarm condition can be suppressed, the existing reference can beautomatically replaced with the new trace, or the operator(s) can bealerted to take manual action. The actual action taken by the system isapplication- and condition-dependent.

In situations where the reference trace is superior, there still existsthe opportunity to extract meaningful data from a statistical comparisonof the traces. In an alarm condition, a one-sample z-test on IL

_(test) will determine whether the alarm is most likely due to a pointfault (line cut or equipment failure) or a cumulative fault (degradedsignal at multiple points). A chi-squared standard deviation computationbetween T_(ref) and T_(test) will give a confidence factor for the alarmitself, with the opportunity to suppress possibly spurious alarms.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the processor performing the statistical calculations included inthe method of the invention is disclosed to be a separate unit form theOTDR unit, those units may be integrated. Further, thespecifically-described statistical techniques may be replaced with otherknown techniques. For instance, chi-squared best-fit technique may bereplaced by least square or a regression fitting technique. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention.

1. A method for analyzing a measurement of reflective characteristics inan optical fiber, the method comprising the steps of: receivingparameters from a test OTDR trace T_(test) of the optical fiber, theparameters including at least one array parameter P

_(test); retrieving from a memory parameters from a reference OTDR traceT_(ref), the parameters including at least one array parameter P

_(ref); correlating fiber characteristics represented by P

_(ref) with fiber characteristics represented by P

_(test) to identify a portion of fiber represented by both P

_(ref) and by P

_(test), and a portion of fiber represented only by P

_(ref); if the portion of fiber represented only by P

_(ref) lies outside endpoints of the portion of fiber represented byboth P

_(ref) and by P

_(test), then triggering a maintenance alert.
 2. The method of claim 1,wherein the step of triggering a maintenance alert is performed only ifthe portion of fiber represented only by P

_(ref) lies outside a far endpoint of the portion of fiber representedby both P

_(ref) and by P

_(test).
 3. The method of claim 1, wherein the step of correlating fibercharacteristics uses an inferential statistical method.
 4. The method ofclaim 3, wherein the inferential statistical method includes performinga chi-squared best fit of the parameter P

_(test) against the parameter P

_(ref).
 5. The method of claim 3, further comprising the step ofselecting a significance level α for the correlation of the fibercharacteristics.
 6. The method of claim 1, wherein the parameter P

_(test) is an array R

of reflectance spikes detected along the fiber.
 7. The method of claim1, further comprising the steps of: receiving a gross distancemeasurement D_(test) from the test OTDR trace T_(test); retrieving fromthe memory a gross distance measurement D_(ref) from the reference OTDRtrace T_(ref); and if D_(ref) is not statistically significantly greaterthan or less than D_(test), and a reflectance array R

_(test) from the test trace T_(test) varies statistically significantlyfrom a reference reflectance array R

_(ref), then replacing T_(ref) in the memory with T_(test).
 8. Themethod of claim 7, futher comprising the step of: if D_(ref) is lessthan D_(test), then replacing in the memory T_(ref) with T_(test). 9.The method of claim 1, wherein the parameter P

_(test) test is an array IL

of instantaneous loss values measured along the fiber.
 10. The method ofclaim 1, wherein the parameter P

_(test) is an array GL

of gross loss values for points along the fiber.
 11. A machine readablemedium containing configuration instructions for performing a method foranalyzing a measurement of light loss in an optical fiber, the methodcomprising the steps of: receiving parameters from a test OTDR traceT_(test) of the optical fiber, the parameters including at least onearray parameter P

_(test); retrieving from a memory parameters from a reference OTDR traceT_(ref), the parameters including at least one array parameter P

_(ref); correlating fiber characteristics represented by P

_(ref) with fiber characteristics represented by P

_(test) to identify a portion of fiber represented by both P

_(ref) and by P

_(test), and a portion of fiber represented only by P

_(ref); if the portion of fiber represented only by P

_(ref) lies outside endpoints of the portion of fiber represented byboth P

_(ref) and by P

_(test), then triggering a maintenance alert.
 12. The machine readablemedium of claim 11, wherein the step of triggering a maintenance alertis performed only if the portion of fiber represented only by P

_(ref) lies outside a far endpoint of the portion of fiber representedby both P

_(ref) and by P

_(test).
 13. The machine readable medium of claim 11, wherein the stepof correlating fiber characteristics uses an inferential statisticalmethod.
 14. The machine readable medium of claim 13, wherein theinferential statistical method includes performing a chi-squared bestfit of the parameter P

_(test) against the parameter P

_(ref).
 15. The machine readable medium of claim 13, wherein the methodfurther comprises the step of selecting a significance level α for thecorrelation of the fiber characteristics.
 16. The machine readablemedium of claim 11, wherein the parameter P

_(test) is an array R

of reflectance spikes detected along the fiber.
 17. The machine readablemedium of claim 11, wherein the method further comprises the steps of:receiving a gross distance measurement D_(test) from the test OTDR traceT_(test); retrieving from the memory a gross distance measurementD_(ref) from the reference OTDR trace T_(ref); and if D_(ref) is notstatistically significantly greater than or less than D_(test), and areflectance array R

_(test) from the test trace T_(test) varies statistically significantlyfrom a reference reflectance array R

_(ref), then replacing T_(ref) in the memory with T_(test).
 18. Themachine readable medium of claim 17, wherein the method furthercomprises the step of: if D_(ref) is less than D_(test), then replacingin the memory T_(ref) with T_(test).
 19. The machine readable medium ofclaim 11, wherein the parameter P

_(test) is an array IL

of instantaneous loss values measured along the fiber.
 20. The machinereadable medium of claim 11, wherein the parameter P

_(test) is an array GL

of gross loss values for points along the fiber.